Thermoplastic aromatic Polyether Ketone derivatives, such as Polyether Ether Ketone (PEEK), are well known to the art. These polymers have melting points greater than 330° C., continuous use temperatures of 260° C. or more and high mechanical strengths, such as tensile strength greater than 85 MPa. They have significant commercial utility as plastics, especially as molded articles and as composites with glass/carbon/Kevlar fibres for a variety of structural applications including in aerospace and general engineering industries. PEEK also finds applications as extruded rods and profiles for manufacture of bushings, seals, etc. In general, they are processed using extruders and injection molding machines in temperature range of 360-400° C., thus requiring extremely high thermal stability.
Literature teaches us two major processes, nucleophilic and electrophilic, for the production of thermoplastic aromatic Polyether Ether Ketone. One is described by Johnson et al, (J. Polymer Sci. 5, A-1, 2375 1967). This nucleophilic route employs hydroquinone and dihalobenzophenone along with a base, in solvents like N-Methyl-Pyrrolidone or Sulfolane, at temperatures of about 200-250° C. The PEEK so produced, however, is found to be of low molecular weight [Inherent Viscosity (Inh. V.)<0.7 dl/g] and cannot be used as a molded plastic due to it's low mechanical properties.
An improvement on this product and process (U.S. Pat. No. 4,320,224/GB 1586 972), involving nucleophilic route is brought about by employing a high boiling solvent Diphenyl Sulfone. In this reaction hydroquinone is transformed into its di-potassium salt by heating with an equivalent amount of potassium carbonate or potassium bicarbonate, with simultaneous removal of the water at 150-200° C., followed by addition of the second monomer, namely, 4,4′-difluoro benzophenone. The polymerization reaction is carried out at 320-350° C. to obtain polymer of desired Inh.V. range of 0.8 to 1.4 dl/g with melting point of 335-350° C. PEEK so produced has structure as well known in the art as given below with two fluoride end-groups.
This process is commercially utilised today. It, however, has several drawbacks. First, it uses expensive raw materials containing Fluorine and Potassium, both of which end up as a by-product to be separated from PEEK. It also requires use of very high temperatures for organic reactions, like 300° C. and above. The use of such high temperature also brings about some charring of material requiring special melt filtration of the PEEK polymer to remove black specs formed during the manufacturing process. The formation of a stoichiometric amount of Potassium Fluoride as a by-product, requires elaborate salt separation procedures to obtain the polymer in pure form. The Diphenyl Sulfone solvent used has a high melting point of 129° C., which makes it inconvenient to process it except at high temperatures. Diphenyl Sulfone is further immiscible with water, hence requiring use of non-aqueous systems for precipitation of the polymer, making its removal from the reaction mass cumbersome.
Hence a process of PEEK manufacturing which can be carried out at lower temperatures, where PEEK can be precipitated in water instead of non-aqueous non-solvents and where recycling of by-products is feasible is most desirable.
Another route for production of thermoplastic aromatic Polyether Ketones like PEEK, involves use of Friedel-Crafts catalysts (electrophilic process). For example, European Patent No. 0174207 teaches the use of AlCl3 for the polymerization of a carboxylic acid chloride derivative of Phenoxy Benzoic Acid (PBA) and Phenoxy Phenoxy Benzoic Acid (PPBA) to give Polyether Ketone (PEK) and Polyether Ether Ketone (PEEK) respectively. The process, though carried out at low temperatures such as 0-30° C., uses AlCl3 in CH2Cl2 solution. Due to the heterogeneous nature of this reaction, generally undesirable lower molecular weight polymers are produced. PEEK polymer obtained by this process is, also, predominantly non-linear and show a high degree of branching. These defects lead to a lowering of the melting point from greater than 330° C. to 315-320° C. There is also reduction of mechanical strength of the polymer formed. It also leads to a significant reduction in its ability to withstand high processing temperatures of 350-400° C. without getting cross-linked. Such a PEEK, therefore, can neither be processed nor be used as a high performance plastic.
Further, the system is highly moisture sensitive due to excess AlCl3 as well as the acid chlorides used as raw materials. Additionally, the precipitation treatment of the reaction mass to liberate the polymer from the catalyst AlCl3 with water involves the liberation of large quantity of HCl gas, which forms effluent. The catalyst AlCl3 used becomes an environmental burden, being non-recyclable and producing huge quantities of effluents containing Al salts. The process itself is also therefore difficult and inconvenient to carry out with no stringent controls for molecular weights.
Another electrophilic process exemplified by Ueda and Oda uses Methanesulfonic acid (MSA)/Phosphorous pentoxide (P2O5) [JOC 38, 4071, 1973, and Polymer 29, 1903, 1983] at low temperatures like 60° C. Inh.V. as high as 1.08 dl/g was obtained. They teach the use of a 1:10 solution by weight of P2O5 in MSA. A mixed anhydride is proposed as the active reagent. While PEEK so produced has less branched structure than one produced using AlCl3 system, it also suffers, like the later, from high temperature instability and hence cannot be molded or extruded without extensive cross-linking and degradation.
Colquhoun has suggested use of Trifluoromethane Sulfonic acid as the reaction medium to polymerize PPBA to give PEEK. (Polymer Preprints, 25, 17, 1984). It has also remained only of academic interest due to the extremely high cost and corrosive nature of the solvent used. Also, in all these above mentioned electrophylic processes, reactive end groups were present. It is part of this invention that PEEK so produced with such reactive groups, like —COOH, present cannot be processed, without end-capping, using traditional plastic processing techniques due to its high thermal instability. Such PEEK on being subjected to high temperature processing immediately cross-links producing gels, which cannot be shaped into desired articles. Therefore, PEEK production by electrophilic processes as described above has not been commercially successful owing to so many inherent limitations involved.
In U.S. Pat. No. 4,247,682 (1981) Dahl has described processes for the condensation of p-phenoxy benzoyl chloride and p-phenoxy benzene sulfonyl chloride in HF using BF3 as a catalyst and using biphenyl or benzoyl chloride as end-capping agents to prepare PEK and PES. These end-capping groups were reported to help maintain the polymer melt stability during extrusion in the absence of which the polymer was reported to degrade readily.
In yet another patent, U.S. Pat. No. 4,808,693 (1989) Dahl, Jansons and Moore have described a process for the condensation of terephthalolyl chloride with Diphenyl ether and diphenoxy benzene using AICl3/EDC system to yield a copolymer of PEKK and PEEKK. Here too, the authors have highlighted the role of the relative ratio of the two electrophilic agents, where higher diphenoxy benzene quantity has increased thermal stability. It may be assumed that use of a given electrophylic system as well as the monomers employed also played a part in determining final structure giving higher thermal stability of the product.
No mention has been made, by the above mentioned authors or to the best of our knowledge by anyone else, for polymerization of phenoxy phenoxy benzoic acid (PPBA) to yield a melt stable and thermally processible Polyether Ether Ketone (PEEK).
In our studies we have found that not only is the nature of the repeat unit critical for obtaining good thermal and mechanical properties, but the nature of the end-group is also critical for attaining desired thermal stability. By manipulating end-groups, it is now possible to prepare electrophilically, novel PEEK structures which show excellent thermal stability and are therefore inherently melt processible.